nitre blue
 

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Rick,
Springfield Armory used the niter process to blue small parts on the early Model 1903 rifles which were not to be heat treated. Their recipe was 100 pounds of refined niter (potassium nitrate or saltpeter) to 10 pounds black oxide of manganese. The parts were suspended above the heated mixture in baskets or on separate wires where the parts could be immersed or removed from the mixture until the correct color was obtained.
Being military parts, the parts were not brought to a high polish before the niter process, but parts which are polished highly will have the peacock blue color which is fairly fragile on the highly polished surface.